This letter directs a colleague to refer a certain kind of client to another company. The letter should simply state that you don't provide the kind of product or service the client needs, and should not rebuke the referring company in any way.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
Thank you very much for referring <referred client> to us. Unfortunately, Abc Company does not provide the proper kind of <products/services> to meet <referred client>'s needs. I have since referred this client to another company more suited to <his/her/their> circumstances.
We at Abc Company certainly have great appreciation for your referrals and continued recommendations, but we don't wish to inconvenience your customers by having them come to us when we can't help them. So if you would, please refer any contacts who need <product or service you don't provide> to <alternative company name> at <alternative company number>. They will look after their needs very well.
Thank you again for the kind referral, and I look forward to the next.
encl: <List of enclosed items goes here>